1. Field of the Invention
The present invention relates to a transfer apparatus, a method of manufacturing the transfer apparatus and an image forming apparatus using the transfer apparatus.
2. Description of Related Art
An image forming apparatus using the electronic photographic method transfers a toner image formed on a toner image supporting body such as a photosensitive belt or an intermediate transfer body, to a recording medium, and melts and fixes the toner image on the surface of the recording medium by using a fixing device. FIGS. 14(a) and 14(b) illustrate a conventional transfer apparatus; negatively charged toner on the photosensitive belt 25 or intermediate transfer belt 19 is transferred to the paper 16 having concaves 17.
FIG. 14(a) illustrates transfer to roughened surface paper as typical inexpensive paper or to a paper surface including concaves 17, such as a second surface deformed by heat generated during the fixing of a toner image to a first surface. The depth d of the concave 17 is 30 to 50 μm, and the width Wh of the concave is 8 to 10 mm. In this transfer, the negatively charged toner 21 needs to be attracted to the paper 16 by an electrostatic field acting between positive charges 20 supplied to the back of the paper 16 by the corona transfer unit 18 and the electrode layer 25b of the photosensitive belt. On the flat part of the paper 16, a toner image is brought into close contact with the surface of the paper 16 and thus a sufficient transfer electric field is applied to the toner 21a, so the toner 21a is efficiently transferred. For the toner 21b facing the concave 17 on the surface of the paper 16, there is a void with a depth of d between the concave 17 and the surface of the paper 16, so the transfer electric field acting on the toner 21b is weakened, lowering the toner transfer efficiency and thereby causing an image failure.
FIG. 14(b) illustrates transfer of a color toner image formed on the intermediate transfer belt 19 to a surface of an embossed paper on which concaves and convexes are artificially formed by performing embossing on coated paper to form a embossed processing such as aventurine lacquer, the texture, the fine grain photoprint. Embossed paper is used to form tickets and front covers of catalogs and brochures. Although the depth d of the concave 17 varies with the type of embossing, the depth d falls within the range of 10 to 30 μm; the width Wh of the concave is 0.2 to 0.4 mm. In this transfer, the color toners of two or three layers formed on the intermediate transfer belt 19 need to be transferred together to the interior of the concave 17, which is narrower than the former concave 17. The transfer electric field is weak for the toner layer facing the concave 17 as in the transfer in FIG. 14(a), and since the image is in color, toners are stacked in a plurality of layers. Accordingly, the transfer electric field is less likely to act on the toners 21a, 22a, 23a, and 24a, which are to be brought into contact with the surface of the intermediate transfer belt 19, further lowering transfer efficiency of the toners 21a, 22a, 23a, and 24a. 
FIGS. 15(a) and 15(b) illustrate forces exerted on toner during electrostatic transfer. In FIG. 15(a), a force is exerted on the toner 21 formed on the surface of the toner image supporting body 38 such as a photosensitive belt or an intermediate transfer belt, when the toner 21 is transferred to the paper 16 by using the corona transfer unit 18. The force by which the toner 21 is attracted to the surface of the toner image supporting body 38 is the sum of a mirror image force FM and van der Waals's force Ff. The force to attract the toner 21 to the paper 16 is an electrostatic force FE based on the positive charge 20 (having a polarity opposite to the polarity of the charge on the toner) supplied to the back of the paper 16.
To overcome the resultant of the mirror image force FM and van der Waals's force Ff so as to transfer the toner 21 to the paper 16, the electrostatic force FE needs to be increased. A method for this is to increase the transfer electric field E by increasing a voltage/current applied to the corona transfer unit 18 so as to increase the corona charge amount of positive charges 20 supplied to the back of the paper 16. If the intensity of the transfer electric field E becomes too high, however, the electric field is locally concentrated and thereby the toner 21 scatters, lowering the image quality. A possible method of solving this problem is to reduce the force to attract the toner 21 to the toner image supporting body 38 (the sum of mirror image force FM and van der Waals's force Ff) and to supply another force to the toner 21 so as to direct the toner 21 toward the paper 16.
The mirror image force FM is electrostatic force acting between the charge on the toner 21 and a mirror image charge generated on the toner image supporting body 38; it depends on the particle diameter and charge of the toner 21 as well as the dielectric constant and thickness of the toner image supporting body 38. The van der Waals's force Ff, which is a non-electrostatic force, is derived from the following equation.Ff=A×R/(6×D2)  (1)
A is the Hamaker constant, which depends on the materials of the toner 21 and toner image supporting body 38. R is the radius of a toner particle. D is a distance between the toner 21 and the toner image supporting body 38. As seen from equation (1), Ff is proportional to the radius R and inversely proportional to the square of the distance D between the toner 21 and the surface of the toner image supporting body 38.
To reduce the force to attract the toner 21 to the surface of the photosensitive body, as shown in FIG. 15(b), an apparatus 39 for vibrating the toner image supporting body 38 is disposed so as to touch the backside of the toner image supporting body 38; when the toner image supporting body 38 is vibrated up and down, an inertia force FB is applied; the sum of FB and FE increases a force to separate the toner from the toner image supporting body 38 so as to move and transfer the toner 21 to the interior of the concave 17 in the paper 16. The inertia force FB depends on the weight of the toner 21, the vibration frequency, and the vibration displacement, as described later. The inertia force FB applied enables it possible to transfer a monochrome toner image (FIG. 15(a)) and to transfer a color toner image comprising a plurality of layers (FIG. 14(b)) to the paper 16 having concaves and convexes on its front surface.
As a means for applying vibration energy from the backside of the toner image supporting body 38 such as a photosensitive belt or an intermediate transfer belt, methods in which an electromagnetic oscillator or ultrasonic oscillator is used are proposed (Patent Document 1). Of these, only the method in which an ultrasonic oscillator is used is put into practical use.
In this method, as illustrated in FIG. 15(b), a horn 39a and an ultrasonic oscillator 39b that uses longitudinal vibration (d33 mode) of a piezoelectric body are combined to structure a resonator with a frequency of 20 to 100 kHz and a vibration displacement of several micrometers; vibration energy is applied to the toner 21 through the toner image supporting body 38 by bringing the vibrating end of the horn 39a into contact with the backside of the toner image supporting body 38 such as a photosensitive belt or an intermediate transfer belt, so that the toner 21 generates an inertia force FB, improving the efficiency of transfer of the toner 21 to the paper 16.
Patent Document 1: Japanese Patent Laid-open No. Sho 55(1980)-20231
Patent Document 2: Japanese Patent Laid-open No. Hei 04(1992)-234076
Patent Document 3: Japanese Patent Laid-open No. Hei 04(1992)-234082
Patent Document 4: Japanese Patent Laid-open No. Sho 62(1987)-248953
Patent Document 5: Japanese Patent Publication No. Hei 04(1992)-20276
Patent Document 6: Japanese Patent Laid-open No. 2005-303937